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I know that active balancing is preferred over its passive counterpart but what active balancing techniques are used in electric vehicles BMS? I also know that switched capacitor and resonant switched capacitor topologies are expensive but passive balancing on the other hand is cheap but has losses. Is the balancing circuit embedded in the chip without energy-storing passive components or what? How are these EV makers dealing with this?

Shamurov
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What type of active cell balancing is used in EV's?

Generally, none.

I know that active balancing is preferred over its passive counterpart

On the contrary: in the majority of applications "charge transfer balancing" is a solution in search of a problem. It's expensive, adds complexity (which reduces reliability) and it has no benefit in most cases (including passenger automotive) because they do not need to balance at high current or faster. In contrast, "Bypass balancing" is simple, reliable, proven, and does the job just fine.

Having said that, there are some specialized applications where charge transfer balancing is advantageous: extremely large batteries, or batteries that are off most of the time.

what active balancing techniques are used in electric vehicles BMS?

Only one-of-a-kind vehicles, such a demonstration sports car, may use charge transfer balancing, not because it's needed, but because it's sexy. Marketing, not engineering.

switched capacitor

That is inefficient because both the cell and the capacitor are voltage sources. A significant portion of the transferred energy is wasted in heat, just due to the physics of connecting two voltage sources in parallel.

Also, it takes theoretically infinite type to fully balance a string of cells because, as the cells equalize, the voltage difference approaches zero and therefore so does the current.

The only charge transfer technologies that work efficiently use inductive components (such as transformers) to store the energy because inductive components can be connected to a voltage source without a problem.

but has losses.

Barely. Like, 1 W per cell. That's about 200 W total for a typical passenger car, and only at the end of charge.

And, "charge transfer balancing also has losses: ~30 % for capacitive balancing, about 3~10 % for inductive balancing.

Is the balancing circuit embedded in the chip without energy-storing passive components or what?

There are very few chips that can do charge transfer balancing. They all use an external passive component for energy storage.

How are these EV makers dealing with this?

By being wise and using plain old bypass balancing.

In case you're wondering: "passive" and "active" are ambiguous terms because they mean different things to different people. That is why a few of us in the battery industry, especially authors, use the non-ambiguous terms "bypass balancing" and "charge transfer balancing".

Davide Andrea
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  • That's not how switched capacitor converters work -- conduction efficiency approaches 100% as voltage drop approaches zero. 50% is only the case for the initial pulse charging from zero to nominal voltage. – Tim Williams Dec 05 '22 at 23:33
  • We seem to be talking different things here; I mentioned conduction efficiency, as in, the electrical efficiency between, in this case, say, +4.2V and -4.2V points (treating the midpoint between two cells as GND, but it might be any midpoint in a pack). Is this the energy flow that loses 50%? – Tim Williams Dec 05 '22 at 23:39
  • @TimWilliams It may be that some implementations of capacitive cell balancing for batteries are 50% efficient, but I don't see anything that makes it inherently so. I have seen plenty of capacitive charge-pump type DC-DC converters that had very high efficiency given the right combination of input and output voltages. – user4574 Dec 06 '22 at 00:57
  • @user4574 Right, it would be rather silly to, say, fully discharge the capacitor after every cycle. I've used charge pumps myself, with efficiency in the high 90s %, many times. – Tim Williams Dec 06 '22 at 05:05